1. Field of Invention
This invention relates to determining image characteristics.
2. Description of Related Art
In conventional marking systems, such as, for example, a laser printer, inkjet printer, or copier, one technique for monitoring the quality of prints is to create one or more “reference” or “test” patches of pre-determined desired tint. The reference/tint of a test patch may be referred to as the call or the density of the reference/test patch. The actual density of the material (often ink and/or toner) in each test patch can then be measured to determine the effectiveness of the printing process on marking a medium, such as for example, a sheet or reference strip. The uniformity of the image on the reference strip can then be determined.
There are many methods known in the art whereby reference/test patches may be used to monitor the quality of prints. For instance, U.S. Pat. No. 6,639,669 B2 to Hubble, the disclosure of which is incorporated herein by reference in its entirety, discloses a color analysis method in which reference strips, with multiple different color printed reference/test patches, are moved relative to a color analyzing spectrophotometer for analysis of test patches and an automatic diagnostic testing of the spectrophotometer. Similarly, U.S. Pat. No. 5,416,613 to Rolleston, the disclosure of which is incorporated herein by reference in its entirety, discloses a calibration arrangement for calibrating a color printer using a plurality of reference/test patches, a densitometer, and a means for converting device independent color information to printer colorant signals, using a look-up table stored in memory.
Likewise, pending U.S. patent application Ser. No. 10/248,390, the disclosure of which is incorporated herein by reference in its entirety, discloses a method and apparatus to calibrate a xerographic print engine toner concentration sensor to control the toner concentration to a specified operating target. This disclosure, includes determining the reflectivities of at least two reference/test patches formed at the same toner concentration, and combining the reflectivities to obtain a combined reflectivity for that toner concentration.
As such, a spectrophotometer, a colorimeter, or a densitometer is generally provided as a tool for evaluating tone reproduction curves and color quality of marking systems. These image measurement devices can measure light reflected from, or transmitted by an object, including an image. These devices use light transmitted and/or reflected by an image to measure the image quality of an image printed on a substrate.
A typical spectrophotometer gives color information in terms of measured reflectances or transmittances of light, at the different wavelengths of light, from the test surface. This spectrophotometer desirably provides distinct electric signals corresponding to the different levels of reflected light received from the respective different illumination wavelength ranges or channels.
Known devices capable of providing distinct electric signals corresponding to the different levels of reflected light received from the respective different illumination wavelength ranges or channels include for example, a portable spectrophotometer as disclosed in U.S. Pat. No. 6,002,488 the disclosure of which is incorporated herein by reference in its entirety.
As used herein, unless otherwise specifically indicated, the term “spectrophotometer” may encompass a spectrophotometer, calorimeter, and densitometer, as broadly defined herein. The definition or use of such above terms may vary or differ among various scientists and engineers. However, the following is an attempt to provide some simplified clarifications relating and distinguishing the respective terms “spectrophotometer,” “calorimeter,” and “densitometer,” as they may be used in the specific context of the specification as examples of providing components for an on-line printer correction system, but not necessarily as claim limitations.
A “spectrophotometer” typically measures the reflectance of an illuminated object of interest over many light wavelengths. Typical prior spectrophotometers in this context use 16 or 32 channels measuring from about 380 nm to about 760 nm or so, to encompass the humanly visible color spectra or wavelength range. A typical spectrophotometer gives color information in terms of measured reflectances or transmittances of light, at the different wavelengths of light, from the test surface. A spectrophotometer desirably provides distinct electrical signals corresponding to the different levels of reflected light from the respective different illumination wavelength ranges or channels.
A “colorimeter” normally has three illumination channels, red, green and blue. That is, generally, a “calorimeter” provides its three (which may be three additive color primaries, such as, for example, red, green and blue or “RGB”) values as read by a light sensor or photo detector receiving reflected or transmitted light from a color test surface illuminated with red, green and blue illuminators, such as three different color LEDs or one white light lamp with three different color filters. A “calorimeter” may thus be considered different from a “spectrophotometer,” in that a colorimeter provides output color information in terms of tristimulus values, such as, for example, RGB and/or related trichromatic expressions, such as, for example, trichromatic coefficients based on tristimulus values. One example of a portable scanning colorimeter is disclosed in U.S. Pat. No. 5,369,494, the disclosure of which is hereby incorporated in its entirety.
Trichromatic quantities may be used for representing color in three coordinate space through some type of transformation. Other RGB conversions to “device independent color space” (i.e., RGB converted to conventional L*a*b*) typically use a color conversion transformation equation, a “lookup table”, or recipe system in a known manner.
A “densitometer” typically has only a single channel, and simply measures the amplitude of light transmissivity and/or reflectivity from the test surface, such as a developed ink or toner test patch, on a photo detector, at a selected angle over a range of wavelengths, which may be wide or narrow. An illumination source, such as an IR LED, a visible LED, or an incandescent lamp, may be used. The output of the densitometer photo detector is programmed to give the optical density of the sample. A densitometer of this type is basically “color blind.” For example, a cyan test patch and magenta test patch could have the same optical densities as seen by the densitometer, but, of course, exhibit different color spectra.
Thus, a spectrophotometer, a colorimeter, or a densitometer may be used as part of an image quality measurement and analysis system to determine image quality problems. These devices, include for example, the X-Rite® DTP41 automatic reference strip-reading spectrophotometer, have traditionally been used to assess raw engine tone reproduction curves, and build calibration look-up-tables, recipes and color profiles. These devices are often co-located with printing systems. Thus, an opportunity exists for extending the application of these devices beyond their traditional functionality.
Traditional use of these devices involves measuring discrete test patches to characterize print engine tonal and color response. For example, in various exemplary embodiments a predefined reference strip containing multiple test patches may be aligned with the device's measurement sensor, the start button pushed, the reference strip driven by the device's drive rollers powered by a motor through the reference strip entrance area of the device, and the reference strip scanned as the reference strip passes the scan area of the device. Measurement values for each reference/test patch may then be reported across a serial interface to a host computer.
It is known in the art that non-uniformity in the appearance of printed materials intended to be uniform is a persistent problem for marking technologies, such as direct-digital production color technologies. Thus, marking machines have inherent error manifesting itself in residual non-uniformities, even after all normal service actions, such as machine self-check diagnostics and technician implemented procedures, have been performed on a marking machine. These residual non-uniformities may occur, for instance, where an image to be printed is intended to be a specific uniform tone, but shows areas which are lighter or darker, or a different tone than other areas. These different areas of the same image are variations that were not intended when the image data was generated and do not reflect the image data generated.
It is known in the art that image quality metrics can be part of an overall image quality analysis engine. For instance, where a region of a printed image is intended to have a uniform color, but shows visible color variations or color differences with respect to the spatial nature of the non-uniformities, the image can be evaluated by a stand-alone scanner, a scanner associated with a printer or a digital camera. The results of the scan can be inputted into an image analysis module. The image analysis module can then quantify different types of non-uniformities and use this analysis to diagnosis printer problems.
For example, U.S. Pat. No. 6,571,000 to Rasmussen, the disclosure of which is incorporated herein by reference in its entirety, addresses the image quality problem relating to regions of a printed image, which was intended to have uniform color, but which show visible color variations. Rasmussen provides a way of evaluating absolute image quality with respect to uniformity, and using the results from the analysis as part of a system for machine diagnostics.
Image density of uniform chromatic and/or achromatic color areas, which may be characterized as image uniformity, is conventionally determined using a particular type of optical device. Image uniformity is conventionally determined using densitometers. On the other hand, image color characteristics are conventionally determined using different types of optical devices such as, for example, spectrophotometers (or spectroradiometers for determining light source color) and/or calorimeters.
For example, U.S. Pat. No. 5,369,494 to Bowden, the disclosure of which is incorporated herein by reference in its entirety, discloses a portable scanning colorimeter. Similarly, U.S. Pat. No. 6,002,488 to Berg, the disclosure of which is incorporated herein by reference in its entirety, discloses a compact spectrophotometer. Both of these patents are assigned to X-Rite®, Inc. Other X-Rite® products are also capable of determining color characteristics of printed materials at various locations on a print medium. For example, U.S. Pat. No. 6,150,062 to Sugizaki, the disclosure of which is incorporated herein by reference in its entirety, determines image density of solid image areas using an X-Rite® 404 densitometer manufactured by X-Rite, Ltd., whereas the color reproducibility is determined with a X-Rite® 968 Spectrophotometer, also manufactured by X-Rite, Ltd.
After data is generated by these devices the data may then be analyzed, applied to algorithms, or otherwise used to determine qualities of the area or image scanned. Pending U.S. patent application Ser. No. 09/941,858, the disclosure of which is incorporated herein by reference in its entirety, discloses methods and systems whereby a spectrophotometer uses an algorithm, based on spectral information of an illumination source and reference spectrophotometer, to convert integrated multiple illuminant measurements from a non-fully illuminant populated color sensor into a fully populated spectral curve using a reference database.
Similarly, U.S. Pat. No. 6,366,362 B1 to Butterfield, the disclosure of which is incorporated herein by reference in its entirety, discloses a procedure for detecting a signal from an device, such as a density photo detector, which monitors each of the individual colors (e.g. cyan, magenta, yellow and black) represented by test patches on a reference strip. After scanning a reference strip, when a particular color is determined to be running at a level above or below a predetermined bit density value, information obtained by the scanning operation is reviewed. When a specific bit pattern or state is detected a template matching process is undertaken, wherein a determination is made as to whether a template matching the scanned image bit pattern exists in storage. When such a template is found to exist, the appropriate template is used in place of a corresponding scanned image area in order to counteract the faulty operation of the printer.
The ability to assess and diagnose unwanted non-uniformity is a problem for field service personnel. Engineering tools such as microdensitometers, two-dimensional precision color scanners, digital cameras, flat bed cameras, and elaborate signal processing which may be available in the lab are generally unavailable to field service personnel who must use simpler and less capable tools. Generally, field personnel must use printed standard image references (SIR) and visual comparisons to determine whether a printing system meets its specified uniformity performance. Additional transparent overlays are placed on printed images to determine spatial frequencies of unwanted image bands. The processes are subjective and thus, have a tendency to be inaccurate.